The present invention relates generally to noise measuring systems, and more specifically to a multichannel phase noise measuring system which amplifies the phase noise of the unit being tested in order to use commercial synthesizers which produce noise levels which exceed that of the tested unit.
Phase noise is a measure of the random phase instability of a signal. Phase noise is commonly measured by frequency analyzers, but commercially available units have limited dynamic ranges and limited frequency bandwidths.
The task of using commercially available equipment to measure multichannel phase noise is alleviated, to some extent, by the following U.S. patents and technical articles, which are incorporated herein by reference:
U.S. Pat. No. 3,535,635 issued to T. Okumura on Oct. 20, 1970; PA0 U.S. Pat. No. 3,711,769 issued to M. Peake on Jan. 16, 1973; PA0 U.S Pat. No. 3,716,785 issued to H. Masters et al on Feb. 13, 1973; PA0 U.S. Pat. No. 3,835,738 issued to F. Edden et al on Sept. 10, 1974; PA0 "Measure Phase Noise In One Of Three Ways" Chuck Reynolds, Electronic Design 4, Feb. 15, 1977, pp 106-108; and PA0 "Low - Frequency Noise Spectrum Analyzer", A. Sal'nichenko, Instrum. & Exp. Tech. (U.S.A.) Vol. 19, No. 5 (Mar 1977) pp 1358-1360.
The above-cited references are exemplary in the art, and all describe noise measurement systems. A common technique entails the use of a mixer to translate the tested radio-frequency signal down in frequency to the range of the frequency analyzer. When using such a technique, the mixer receives both the radio frequency signal which is being tested, and a reference signal from a phased-locked signal source. The mixer outputs an intermediate frequency signal which is processed by a low-pass filter and analyzed by a frequency analyzer. The problem with this approach is that the reference signal source must have better phase-noise charateristics than the unit under test.
A critical need of a doppler radar system is a low phase noise output. Measuring such low phase noise levels can be difficult, particularly when the commercial test equipment has phase noise levels higher than that of the unit to be measured.
In view of the foregoing discussion, it is apparent that there currently exists the need for a system capable of amplifying the phase noise of the unit being tested in order to use commercial synthesizers having noise levels exceeding that of the unit being tested. The present invention is intended to satisfy that need.